lchthyopsida 



427 





Ji<:', 'y'^t iifs?-* > y 



X 



Fig. 330. (Top) Rhinobatus lentiginosus (Garman) : in form of body, inter- 

 mediate between an ordinary sbark and the very broad thin skate. (Bottom) Raja 

 erinacea (Mitchill). (Courtesy, Jordan and Evermann: "Fishes of North and 

 Middle America," Washington, The Smithsonian Institution.) 



connection with the pharynx. This air-bladder may serve as an acces- 

 sory respiratory organ (when open into the pharynx) or it may have a 

 hydrostatic function, making possible the adjustment of the specific 

 gravity of the fish to that of the water by changes in the volume of gas 

 within the air-bladder. In some fishes the organ probably serves both 

 functions. The gas within the closed air-bladder is mainly oxygen de- 

 rived by diffusion from blood contained in vessels in the wall of the 

 organ. The volume of gas in the bladder may, on occasion, be reduced 

 by absorption into the blood. 



The following subdivisions of Actinopterygii are generally recog- 

 nized. 



Order Chondrostei 



Notochord fully persistent; vertebrae represented only by carti- 

 laginous or partly bony arches — no centra. Skull largely cartilaginous. 



